Method of isotope analysis



Patented Feb. 19, 1952 UNITED STATES PATENT GFFICE METHOD OF: ISOTOPE ANALYSIS Gerhard ILDieke, Baltimore, MgL assignor to the United States of America as represented by the Uhi te tl states Atornic Energy Commission No Drawing.- Application Nove nberzl, 1945,

ss ihl who cens s (ems-230) Thisgmv t h u elat g o o e analys s ahh xnoraparticularly to anovel spectrometric.moth

osko eter n n he-i otop ch c it o rhi an lemen n :rec h she e as been ohsi e ab a r terest in processes-v tor separating, the isotopes of v r-io src em l m ntsv Mus Qf hi in e has centered; on; methods for separating. the. isotepes of v uranium to obtain the uranium isotope o f mass, 235- for use in, the. production of atomic energy, and numerous methods of separating the uraniumisotopeshavebeenproposed. There has also een' nt h arious m tho sl r n the natural isotopic abundance.oihother elements such as oxygen nitrogen angl .canbonr These and other elements-when enriched withrespect to art c lart s. ma o sed, o amp e, as tracers in physiological; studies and. for other. pur o es:

As; an incident of such isotope separation and concentration processes it is frequently; desirable to; make analyses, of; isotopic compositionbe-. cause of the fact that. suchanalysesareuseiulin achieving efficient operation and efiectivevcontrol f; he a i n: nd a c nc n ration ess s- Thgi problems. involved in performing an accurate isotope analysis are, toa considerable extent analogousto the, problems encountered, in per-. forming; an isotope. separation. Since isotopes are merely difierent forms of ;the same chemical element, conventional techniques on chemical analysis cannot beeffectively used, Eurthermore the isotopes of anelement generally resemble each-other-closely in their physicalcharacteristih ran he fore t pr b m f e mi an aocurateiso-tope analysis is at-very difiicult one.

It, is an object of the-present invention; to pros Vide; a simple, efiectiye-and accurateimethod -of de minih he: m mmaompo ition ahe: men

Itis n he e t i 1 BYfiRfiQhiQ-PIOVM? a novel spectrometricmethod-oi isotope analysis, 1. e.-, a, method wherein the isotopic-,composition ia e en s et in dr mhi cor s ondin spec ral; es: hara eristi 01.1 1? s op s of the element;

O her bjectsof the, inve i n: i b 1 art b ious. n p rt in ed ut, here f n; on -Q oader; asnecta the pres h 1hh qh om ris s. a method ihd cnmi he the sotop a mpos ions: anl lementhy i QIDQ cl fa ap eitohn oh 11 hirhl heshot ft e. 1%; o substance m ht. RIQQWiP-g auspestruin hslhhih .su hn orres oh ih in s an Obtaining r mlth ho resn nhih nes o her hss h mgication of the isotopic cornposition'of the ele-; ment. As conducive tqa clearer understandin f v met od. fth n en it a e oin d o t hat chemi al suh ta he a absorb '0 emit radiation only at certain energy levels and that the wave lengths of such omitted or absorbed ad on are. cha ac eris the; truc an om osi ion, or uch, huhn e Al hh h so opes o an. leme ar uhs oah a v den i in many oi their. characteristics' ancl properties they ncrn allyv differ somewhat in the structure and composition of their atoinic nuclei. More sn cifihall i ot pes ordinaril i i h h" W h cts hi h hfl hh e-ihe hhl Q h m1 dih qh the ab o vQ hl li. 8- mas l d e r in? hus e hv e h oi hef h' i tion absorbed or emitted by a given element inay vary li tly, w h ts s o c. em b h'afi ence the PfiQ Il F of he element il fsX qr eseonshhe siif .rhi es-v The e f er n mars pwhh n he. s ectrum s a iehle emhfi i. curesh nd os lines at rihhthhl to h Q frhhtisot ess of. he l m n f h hliih eterm nation o ir 'q op e omp siti n s re he lelaerm hti hnmhy. be as? by me el qb ervmeh hidehhiy n he I h shefl acterisliq ith r ihhr h ot es I 9 51,1 11? h a cL, a uant tat ye e ermihah h, is i l red, av m a r ment i herelh iv n s me ases he b olute nt ns s. f, the, pe a lin snia be, ak n a h me shre H e l l ahim nhe of he s ffisr ht 9 QP hreisht 9??? lement. i

n attem t n to mak u h a EEKQWQQPQ f sotopic com s t on l mlhv he di l c men Q? ha oeoh or corre ond n shehtral i hs a pi he le h risesp itoi the hht h h h ihel hes-i h fl sp acemshi f9! d sr hlishtehs an .e jemen ss9 s allies QPlFQWlhQFQEQ'? resa e; o secondaries. w thth'e resent meFhQQ this 5 fich i s srcome b h r h h i s the lemen oiuh sh wh. soto ic om s ioh.ihlas ib ia ually al he ic l, QPIIJPQRBQ. w i h s of? .ehh; hihehhh ra oh Qi hQPQI Q RQhQ E s o rallin s haract rist c llh fifii 2 P11? leins i-. thas been f uh en pe amhla that ar ous liall ll alts. a e a ab e .q i hers i l 1? es natiq f. prresh hhih p ct a l e QR? the is topesqf e ehl hts hohta hqd in bhell hhy salt ansl thy n or ora n leme s a uranium! oxy en and. tt'qsehn, h. as h hie hra x sa t ispl me r ine or a ,g ogs'lip es the absorption 'andffiuorescence accordingly the present method is preferably car- 1 ried out at temperatures of the order of 20 K. to

80 K. It should be understood however that this temperature range is not critical and that satisfactory results may be obtained at temperatures not within the stated range.

As previously indicated, the present method may be used for either qualitative or quantitative isotope analysis. When the method is used to obtain quantitative results a number of alternative procedures may be used. Thus the relative abundance of the isotopes of an element may be determined by determining the relative intensities of corresponding spectral lines and more particularly by determining the ratio of the intensities of a pair of corresponding lines as a measure of the ratio of the concentrations of isotopes producing those lines. In other cases, as for example where a large number of similar determinations are to be made, the absolute intensity of one line may be measured by a suitably calibrated light responsive device to indicate the concentration of the particular isotope producing that line. In still other cases it may be desirable to compare the intensity of a particular line corresponding to a particular isotope with a line produced by a sample of known isotopic composition to determine the concentration of the particular isotope in an unknown sample.

Various methods of measuring the intensity of spectral lines may be used and the present invention is not limited to any particular method. Visual inspection of the spectrum gives a rough indication of relative intensity and hence composition although it will be apparent that such visual inspection is unsatisfactory where precise quantitative results are desired. A more accurate determination may be obtained by photographing the appropriate section of the spectrum and determining the relative intensities of the desired lines from the photograph. This method' although more accurate than visual inspection is tedious and still leaves something to be desired where precise results are sought. More accurate results may be obtained in a rapid and efiicient manner by the use of a photoelectric apparatus of the type disclosed in an article by Dieke and Crosswhite in the Journal of the Optical Society, vol. 35, No. 7 pages 471 to 480, July 1945. In accordance with the method disclosed in this publication a light sensitive tube such as a photomultiplier tube is positioned to receive light through a slit of predetermined width located at the focus of the spectral line or lines of which the intensities are to be measured. The tube is preferably so mounted as to scan the spectral lines and the output of the tube is transmitted either directly or through an intermediate amplifier to a suitable meter, for ex ample, a recording microammeter. Since the current produced by the tube is a function of the intensity of the spectral line being scanned the record on the microammeter indicates the intensity of the line and hence the abundance of the isotope producing the line. Since there are many factors such as variations in the light source, variations in the condition of the sample etc. which tend to alter the absolute intensity of the spectral lines it is ordinarily desirable to compare the intensity of a pair of corresponding lines thus eliminating errors .of this character. When using the photoelectric apparatus referred to, the ratio of the intensity of a pair of corresponding lines may be determined either by measuring the individual intensities of the lines and computing the ratio or preferably by connecting a pair. of phototubes responsive to the "different line intensities in the well known Wheatstone bridge circuit so that the ratio of the two intensities may be measured directly.

In cases where a'uranyl salt is used to enhance the separation of corresponding spectral lines the isotopic abundance of a number of elements may be determined simultaneously. Thus isotope analyses of uranium and oxygen may be made using any uranyl salt. Uranyl nitrate may be used in theiisotope analysis of nitrogen, uranyl chloride in the case of chlorine, and uranyl carbonate in the case of carbon. Other uranyl compounds. may be used in the analysis of still other elements where such elements can be incorporated in a suitable uranyl compound.

The spectrum used may be either an absorption spectrum or a fluorescence spectrum. In general the fluorescence spectrum is preferable since it may be effectively obtained with the salt in relatively finely divided crystalline condition thus making it unnecessary to go through the procedure of preparing large precisely formed crystals. In determining the isotopic composition of oxygen or nitrogen the fluorescence spectrum gives satisfactory'results. In the case of uranium, however, the displacement of corresponding lines in the fluorescence spectrum is so small that it is preferableto use the absorption spectrum. Methods ofpreparing the large crystals required for obtaining such an absorption spectrum are-known in the art.

In order to point out more fully the nature of the present method the following specific example is given of a procedure by means of which the abundance of the oxygen isotopes in an oxygen sample of unknown isotopic composition may be determined. A sample of cesium uranyl nitrate CSUO2(NO)3 was prepared by incorporating in uranyl nitrate a sample of oxygen and crystallizing the mixed salt from a solution containing cesium nitrate and the uranyl nitrate as thus prepared. The oxygen may be incorporated in the uranyl nitrate by conventional methods. For example, nitric acid may be prepared by causing the oxygen sample to react with ammonia and the resulting nitric acid used to dissolve uranium-metal to produce uranyl nitrate. Relatively small crystals of the double salt were placed in a small tube which was inserted in a larger double-walled tube containing liquid hydrogen. The double-walled tube functioned in the manner of a Dewar flask to prevent excessive vaporization of the hydrogen and the hydrogen served to keep the sample of cesium uranyl nitrate at the desired low temperature. The sample was exposed to a suitable source of light, in this case an are light and radiation from the sample was focused on the slit of a conventional spectrograph. In order to obtain the desired dispersion of the spectral lines it is desirable to use a spectrograph having a relatively high resolving power. In the present instance the spectrograph used had a asse sor 2'1-foot*-{ diffraction -gra-ting a Wadsworth lb niiii f wfiichfisav ai i i r 39 stromjsxmm: At; the focus-of ithe-"--spectrumas producedtbythis-instrument there; was a -second slit* oj a djustablewidth behind which there was aph'otomultiplier tube of the' type previously 're ferredto". Itiis amatterfof 'some' importance 'in obtaining accurate resultsthat the width-of this second slit be properly adjusted-and in general the -slit' shou-ld beadjusted to the width of the widest line thatis to be scanned by the phototube. In the present instance aslit width of 0.25 mmaisyused.

In the fluorescence spectrurngof cesiumruranyl n trate there area, number. of, pair of corre- DDnding J Spmduced bythe uranv fsro an having displacements and intensities of's'uch magnitude as to be satisfactory for determining the isotopic composition of the oxygen contained in the group. of the sample is small the number of molecules containing UO O groups is negligible and hence the significant spectral lines will be those produced by UO O groups and UO O groups. A convenient pair of lines to use under these circumstances are the lines identified by the wave numbers 20205.58 cm.- and 20242.69 cmr The former line is characteristic of the UO O group and the latter line, which is displaced 37.11 wave numbers toward the blue end of the spectrum, is I characteristic of UO O Since the 0 content of present sample was small the intensity of the [70 0 line was much less than that of the UO O line.

These two lines were scanned by the phototube and the current generated by the tube was amplified and recorded by a recording microammeter. The ratio of current intensities obtained was 37, i. e., the current generated when the phototube viewed the T0 0 line was 37 times as great as when the tube viewed the 10 0 line. This current ratio indicates that the sample contains 37 UO C) groups for each UO O group and thus contains 2X37+l=75 O atoms for each atom of C Hence the C content of the sample is about 1.34% of the 0' content of the sample.

In order to determine the accuracy of the foregoing method, an oxygen sample known to contain 1.36% O was incorporated in cesium uranyl nitrate as described above and analyzed by the foregoing method. The current density ratio was found to be 37 as before, thus indicating an 0 content of 1.34% and demonstrating that the present method is accurate to about 2%.

It is to be understood that the foregoing description is illustrative only. In cases where the isotopic composition of uranium is to be determined the degree of separation between analogous lines that is obtained in the fluorescence spectrum is quite small and therefore it is preferable to use the absorption spectrum. Thus a single, large crystal of cesium uranyl chloride CszUOzCh may be inserted in the sample tube as described in the foregoing example and subjected to light source which in this case may be a lamp having an ordinary tungsten filament. The radiation from the sample is preferably polarized before it passes to the spectrograph. Separations between analogous lines produced by U O2 groups and [7 02 groups of the order of 1.6 to 1.8 cmr have been obtained.

Since many embodiments might be made of the present invention and since many changes might be made in the embodiment described, it is In cases where the 0 content to beunderstood that the Jore'goi'ng descriptionin-a limitingsense.

'-I 'claim:- X

1 A method of determining: the lisot'opicicomposition of an element capable of. being .incor porated in a-'-uranyl compound which .ucomprises, chemically incorporating saidelement in a-uranyL compoundto enhance' thef I separation of corre-i. sponding spectral lines characteristic 01 =the..iso'-. topes of said element, producing a spectrum 01.; said compound including said corresponding lines and scanning-at least one ofsaid lines with' a light sensitive tube to obtain: an electric cur-' rent as anyindication or -the relative abundance is to be interpreted as illustrative only andmot;

ofjsaid isotopes in said element. 5 v

2: A method of determining the-isotopic com-., position of an element capable of being incorporated in a uranyl compound which comprises chemically incorporating said element in a uranyl compound to enhance the separation of corresponding spectral lines characteristic of the isotopes of said element, producing a spectrum of said compound including said corresponding lines and scanning at least two of said corresponding lines with a light sensitive tube to obtain a plurality of electric currents, the ratio of which serves as a. measure of the relative abundance of said isotopes in said element.

3. A method of determining the isotopic composition of an element capable of being incorporated in a uranyl compound which comprises chemically incorporating said element in a cesium uranyl salt to enhance the separation of corresponding spectral lines characteristic of the isotopes of said element, producing a spectrum of said salt including said corresponding lines and scanning at least one of said lines with a light sensitive tube to obtain an electric current as an indication of the relative abundance of said isotopes in said element.

4. A method of determining the isotopic composition of an element capable of being chemically incorporated in a uranyl compound, which comprises incorporating said element in a uranyl compound to enhance the separation of corresponding spectral lines characteristic of the isotopes of said element, producing an absorption spectrum of said substance including said corresponding lines and scanning at least one of said lines with a light sensitive tube to obtain an electric current as an indication of the relative abundance of said isotopes in said element.

5. A method of determining the isotopic composition of an element capable of being chemically incorporated in a uranyl compound, which comprises incorporating said element in a uranyl compound to enhance the separation of corresponding spectral lines characteristic of the isotopes of said element, producing a fluorescence spectrum of said substance including said corresponding lines and scanning at least one of said lines with a light sensitive tube to obtain an electric current as an indication of the relative abundance of said isotopes in said element.

6. A method of determining the isotopic composition of oxygen of unknown isotopic composition which comprises chemically incorporating said oxygen in cesium uranyl nitrate to enhance the separation of corresponding spectral lines characteristic of the isotopes of oxygen, producing a fluorescence spectrum of said nitrate including said corresponding lines and scanning at least one of said lines with a light sensitive tube to obtain an electric current as an indica- 7 tionof the relative abundance of said oxygen isotopes. a

7. A method of determining the isotopic composition of nitrogen of unknown isotopic composition which comprises chemically incorporating said nitrogen in cesium uranyl nitrate to enhance the separation of corresponding spectral lines characteristic of the isotopes of nitrogen, producing a fluorescence spectrum of said nitrate including said corresponding lines and scanning at least one of said lines with a light sensitive tube to obtain an electric current as an;indication of the relative abundance of said nitrogenisotopes. I

-8. A- method of'determining the isotopic composition of uranium of unknown isotopic composition which comprises chemically incorporating said uranium'in cesium uranyl chloride to enhance the separation of corresponding spectral lines characteristic of the isotopes of uranium, producing an absorption spectrum of said chloride including said corresponding lines and scanning at least one of said lines with a light sensitive tube to obtain an electric current as an indication of the relative abundance of said uranium isotopes.

GERHARD H. DIEKE.

REFERENCES CITED The following references are of record in the file of this patent:

Satyanarayana, Fluorescence of Uranyl Salts, Chemical Abstracts, vol. 38, p. 6200 (1944). 

1. A METHOD OF DETERMINING THE ISOTOPIC COMPOSITION OF AN ELEMENT CAPABLE OF BEING INCORPORATED IN A URANYL COMPOUND WHICH COMPRISES CHEMICALLY INCORPORATING SAID ELEMENT IN A URANYL COMPOUND TO ENHANCE THE SEPARATION OF CORRESPONDING SPECTRAL LINES CHARACTERISTIC OF THE ISOTOPES OF SAID ELEMENT, PRODUCING A SPECTRUM OF SAID COMPOUND INCLUDING SAID CORRESPONDING LINES AND SCANNING AT LEAST ONE OF SAID LINES WITH A LIGHT SENSITIVE TUBE TO OBTAIN AN ELECTRIC CURRENT AS AN INDICATION OF THE RELATIVE ABUNDANCE OF SAID ISOTOPES IN SAID ELEMENT. 